Ferrous metal container for ammonium nitrate solution and method of reducing corrosion thereof



FERROUS METAL Patented Feb. 26, 1963 CONTAENER FOR AMMO l NITRATE SOLUHON AND METHOB F RE- DUCING CORROSION THEREOF David B. Sheldahl, Grifiith, Ind., and Lawrence V. Collings, Harvey, Paul Shapiro, Chicago, and Franklin M. Watkins, Flossmoor, IiL, assignors to Sinclair Research, Ind, Wilmington, Dei., a corporation of Delar ware N0 Drawing. Filed Nov. 1, 1961, Ser. No. 149,188

6 Claims. (Cl. 206-84) The present invention relates to the corrosion inhibition of ferrous metals. More specifically, the present invention relates to a composition and method of reducing the corrosion of ferrous metal surfaces caused by contact with aqueous ammonium nitrate solutions.

There is a Well recognized corrosion problem in industries concerned with the manufacture, storage, transportation and handling of aqueous ammonia ammonium nitrate solutions. In the handling of such solutions it is convenient to transport and store them in ferrous containers such as drums, tanks and pipelines. However, in view of the corrosive nature of ammonia ammonium nitrate solutions on ferrous metals many manufacturers now use storage and transportation facilities constructed of aluminum. Aluminum is used because its oxide film renders the metal inert to attack by the ammonium nitrate solutions. The remedy, however, is a costly one. Corrosion inhibitors of one type or another have been suggested and attempted with varying degrees of limited success.

One answer to this corrosion problem presented by ammonia ammonium nitrate solutions is the utilization of coating materials so as to exclude the salt solution from the ferrous metal surface. This technique has not been employed in ammonia ammonium nitrate solution service to any great extent for if any pin holes remain in the coating after application, the salt solutions attack the metal surface at these points. The large amount of corrosion product fromed in these areas causes the rest of the coating to peel and eventually the entire ferrous metal surface is exposed to the corrosive solution.

Copending application Serial No. 149,186 in the names of Franklin M. Watkins and David B. Sheldahl, filed November 1, 1961, concurrently herewith discloses a relatively thin coating composition consisting essentially of minute particles of carbon in a binder chemically inert to ammonia ammonium nitrate solutions, which composition remedies the aforementioned problems and provides effective corrosion reduction. The effectiveness of the compositions of that application is due to the action of the carbon in inducing the formation of a gamma-R 0 film on ferrous metal surfaces exposed by pin holes that develop in the coating or are initially present after application of the coating. This oxide film passivates the area and prevents attack by the ammonium nitrate solutions. Formation of this passive oxide film is due to the fact that the electrical potential existing between the exposed ferrous metal and the carbon particles contacting ammonia ammonium nitrate solutions is sufiicient to create the current density and voltage required for passivation to occur. In NH --NH NO solutions ordinarily 1.5 ma./cm. is required for passivation and the resulting (working) voltage will generally be at least about 0.7, usually 0.7 to .75 volt.

We have now found that a passive film of increased stability and therefore a coating composition of improved corrosion resistance can be obtained by incorporating in the carbon-containing coating composition a minor effective amount of an arsenate or arsenite insoluble in ammonium nitrate solutions and essentially soluble in nitric ferrous surfaces,

acid. The preferred arsenic compounds are the arsenates. Examples of suitable arsenic compounds for use in the present invention are, for instance, lead, barium and zinc and other heavy metal, i.e. atomic number of at least 25, arsenates or arsenites such as arsenic trioxide etc. The amount of the arsenic compound incorporated in the coating composition will vary depending on the particular ammonium nitrate solution employed but will ordinarily fall within the range of between about 1 to 20% by weight, preferably at least about 10%.

The carbon employed in the coating composition can be carbon in its amorphous or crystalline forms, for example, charcoal or graphite and is present in a pulverized or finely-divided dispersible state. Generally, the size of the carbon particles will fall in a range of about 10 to 200 microns, preferably 20 to 50 microns, in diameter.

These carbon particles are in direct contact with the preferably sufiicient of the carbon particles are in direct contact with such surfaces to give the desired galvanic protection. Accordingly in the thin coating say of about 0.5 to 20 mils, preferably about 1 to 10 mils, thickness, in electrical contact with the ferrous surfaces, there is generally at least about 5% by weight of the coating of the carbon particles, advantageously at least about 65% by weight, more preferably about 70 to by weight. The amount of carbon particles necessary or desired is dependent upon factors such as their surface area, the extent of their electrical contact with the ferrous surfaces, the rate of contacting the ammonium nitrate solutions and the presence of other ingredients, e.g. corrosion inhibitors, etc. in the coating. Thus only those particles in such electrical contact and in electrical contact with the ammonium nitrate solution are providing the desired galvanic protection and effectively preventing corrosion. A given particle may not directly contact both the ferrous surface and nitrate solution but the contacts may be established through intermediate electrically conductive material such as other carbon particles.

If the viscosity of the binder component will permit, the carbon and arsenate or arsenite may be incorporated directly in the binder component by slow addition and stirring. Preferably a suitable solvent, for instance a its viscosity and facilitate dispersion of the carbon particles and arsenic salt. After the carbon and arsenic compound are thoroughly mixed with the hinder, the solvent is then removed as by evaporation to produce the coating composition. By coating composition, we mean a composition that is directly self-adhering when applied to a surface.

The binder component of the coating composition can be any of the materials commonly employed as ,binders in paints and other coating products which are chemically inert to ammonium nitrate solutions and may constitute the balance of the composition generally from about 10 to 40% by Weight. Examples of suitable binder material are: organic resins such as vinyl copolymer resins, epoxypolymers, styrenebutadiene copolymers, vinyl chloride resins, polyurethane, oil-treated isocyanates, acrylic resins etc. Inorganic binders such as the silicates may also be used in some instances.

The carbon particles and arsenic salts are held in coating position by an organic or inorganic binder such as an organic resinous material, paint or similar coating composition. Also the carbon-containing coating can be overlain by another coating composition if desired, although this is not necessary, and it is when the carbon particles in electrical contact with the ferrous surfaces are also in contact with the ammonium nitrate solution that the corrosion inhibition is being afforded by the galvanic apropos couple. Contact with the solution may be through carbon particles at the outersurface of the coating or at holes or cracks in the coating which are present due to imperfections in the coating, wearing away of the coating, etc. A carbon particle'in direct contact with another carbon particle in the coating can be considered as a single particle. a Y

The coating compositions of the present invention may also include other ingredients which are not deleterious to the passivating activity of the carbon particles, for instance ingredients commonly found in pain-ts and other coating products as, for example, metal oxide'pigments, oils, plasticizers, resins, etc. In actual practice it may be found advantageous to just incorporate the carbon particles in, for example, a paint composition commercially prepared whose formulation includes a suitable binder for the carbon particles particularly those formulations noted for their superior abrasion resistant qualities, their strong adhesion to ferrous metal surfaces, and inertness to ammonia ammonium nitrate solution.

The ammonia ammonium nitrate solutions may vary considerably in composition. Although my system protects vessels containing aqueous ammonium nitrate solu- 'tions greater need and utility residesin protecting vessels employed to'handle aqueous ammoniacal aqueous ammonium nitrate solutions. Generally representative of such solutions encountered in 'industry'and which give rise to the corrosion problem discussed heretofore are those having approximately about 1 to 80 or more percent ammonium nitrate usually at least about 40 percent, preferably about 60 to 70 percent, about 5 to 35 percent free ammonia, and the substantial balance being water, for instance, about 5 to 25 percent water. These percentages are by weight. Especially useful solutions are those containing a ratio of free ammonia to water of at least about 1.5 to l by weight.

Even greaterassurance of full protection for the metal surface can be obtained by employingin addition "to the coating composition of the present invention, a fer rous metal-carbon galvanic couple, that is, employing one "or more elemental carbon cathodes in aqueous ammoniacal solutions and the ferrous metal as the anode and having a metallic conductive path between the two. Thus a ferrous metal-carbon cathode galvanic couple is established which generates'sufiicient current in situ, that is, without the use of an external current source to accomplish passivation of'any areas of the ferrous metal that for some reason are not passivated by the coating composition of the present invention. More important, the ferrous metal-carbon galvanic couple supplies sufficient current to assist the maintenanceof passivation acquired from the coating composition of the present invention.

The current density created by the ferrous metalcarbon galvanic couple is dependent on the surface area ratio of the active ferrous metal to carbon in contact with the solution. The active ferrous metal is the non-passivated ferrous surface in contact with the solution and thus the rate of addition of the ammonium nitrate solution may affect the extent of corrosion protection obtained. Accordingly the surface area ratio of ferrous metal to carbon and/ or rate of addition should be selected to produce a current density sufficient to passivate the ferrous surface. For example, the current density can be increased by increasing the area of the carbon or slowing the rate of filling of the container. When the ratio of surface area of the ferrous metal to carbon is about 1:1 to 15:1, for instance, it has been found that sufficient current density is generated even to passivate ferrous metal containers to which the ammonium nitrate solutions are rapidly added. 1' Much greater ferrous metal to carbon surface areas, say even up to 200 or even more: 1, can be used when the ammonium nitrate solution contacts the metal slowly. Ordinarily, a current density in the range of at least about 0.1 to about 1.5 or more ma./cm. is found sufficient for passivation of the ferrous metal and the resulting voltage will generally be about 0.7 to .75

volt.

Although the carbon cathode can be suspended into the aqueous ammonium nitrate solutions it is preferred that it be directly connected to the lower portion of the container, e.g. the bottom particularly in the case of containers of large-size, such as storage tanks so that the current density generated by the couple is sufiiciently great to passivate the ferrous metal as the solution contacts it. This construction maintains the carbon electrode in contact with the nitrate solution in the vessel. It has been found that connecting the carbon cathode, preferably a plurality of carbon cathodes to the bottom of a large container such as a tank car, required steelzcarbon surface ratios are assured and passivation is accomplished.

If desired, chromate salts or manganese compounds e.g. Mn0 insoluble in ammonium nitrate solutions can also be added to the coating composition of the present invention in minor effective amounts. These compounds have been found advantageous tlI'l eliminating the possibility of a reduction in (working) voltage between the anodic pinhole area and the cathodic carbon surface so that voltage values are reduced below that required for passivation. Addition of these compounds is also advantageous in that the compounds have been found to act as passivatingagents for ammonium nitrate solutions. Generally the amount of chromate salt-or manganese compound incorporated will fall in the range of about 5 to 20 weight percent.

The ammoniacal solutions may contain additives well known to the art as corrosion inhibitors in these solutions. Examples of these inhibitors are trivalent arensic compounds, for example, arsenic trioxide; an arsenite such as sodium, potassium or ammonium arsenites and sulfides of trivalent arsenic; compounds which contain divalent sulfur linked to an atom of carbon with the remaining valences of the carbon atom linking the carbon cyanates, thio'carboxylic acids, thioamides, etc. (see US. Patent No. 2,220,059 to-Herman A. Beekhuis et a-L); and organic compounds having an SH and an OH group, for instance, as disclosed in U.S. Patent No. 2,613,131 to Marion D. Barnes et al. In fact the presence of these additives, particularly the arsenites, when ferrous metal surface is being passivated by the method of the present invention may enhance the passivation even in amounts far smaller than taught as efiective by the prior art. The presence of compounds which provide theammoniacal solution with copper and carbonate ion, for instance, basic cupric carbonate, have also been found to enhance the passivation. Not only is'the presence of these additives of advantage in enhancing passivation but once passivation has been accomplished they act to further insure protection.

a A particular effective inhibitor additive is the combination of trivalent arsenic compound, asoluble copper compound and carbonate ions as disclosed in application Serial No 5,637 in-the names of Paul Shapiro, David B. Sheldahl, and Lawrence V. Collings, filed February 1, 1960, herein incorporated by reference. The soluble copper compound can be, for instance, the inorganic compounds such as cupric carbonates, hydroxides, sulfates, nitrates, etc. Of the many carbonate ion-producing compounds, the more particularly suitable are the inorganic compounds, for instance, alkali metal and ammonium carbonates. Preferably, the copper and carbonate components are provided by a single compound such as basic copper carbonate.

The following examples are included to further illustrate the present invention:

EXAMPLE 1 Steel panels 1" x 5" x 1 /1 were cleaned by sand blasting and coated by dipping into the compositions indicated in Table I below. Coatings were made by suspending atom to nitrogen as, for instance, carbon disulfide, thiographite (325 mesh) with and without an arsenate or arthe arsenate or arsenite was activated in a matter of secsenite in a binder also identified in Table I below. onds. Composition H, the arsenate-containing coating After proper curing time (about a week) the coatings composition of the present invention, required over 2 were gouged with a sharp file, inducing two scratches in hours to be activated. Composition I, the arsenite-conthe criss-cross pattern extending the length of the coupon. 5 taining composition took 30 minutes. The couplo-ns were then placed in eight-ounce French It is claimed: square bottles partially filled with about 100 ml. of aque- 1. A method for reducing corrosion of ferrous metal ous ammoniacal ammonium nitrate solution so as to surfaces by aqueous ammonium nitrate solutions which allow about half the coupon to extend above the liquid comprises coating said ferrous metal surfaces with a coatline. After about a day or two, appearance and single ing composition consisting essentially of at least about electrode potentials indicated that the coupon was pas- 5% by weight of minute carbon particles, about 5 to 20% sive, some of the coupons were removed and still wet with by weight of an arsenic compound selected from the group solution, the untouched side was gouged in the manner consisting of arsenates and arsenites essentially insoluble described above. The rescratched coupon was then imin said ammonium nitrate solution and essentially soluble mediately reimmersed in the test solution. The aqueous in nitric acid, the essential balance being a binder chemiammoniacal ammonium nitrate solution contained 66.8% cally inert to said ammonium nitrate solution, and con- NH NO 16.6% NH;, and 16.6% H O. The results are tacting said coating with an aqueous ammonium nitrate shown in Table I below: solution.

Table l Percent pigment Coating No. Percent binder Observations Carbon Other G 74 13BaCrO4 13-lacquer Scratched prior to immersion. No corrosion. Removed from H 74 13Pbw(OH)tO (AsO4) do solution, scratched while wet, returned to solution-mo corrosion for over 3 months. I 71 9AS:O3 -vinyl chloride Scratched prior to immersion, no

resin. corrosion.

2. The method of claim 1 wherein the solution is ammoniacal ammonium nitrate.

3. The method of claim 2 wherein the arsenic compound selected is lead arsenate.

4. A ferrous metal container containing an ammonia- The data of Table I demonstrate the corrosion inhibiting properties of the coating composition of the present invention.

The resistance of the passive film to destruction can be observed by recording the decrease in potential when a metal anodic to the passive steel is brought into electrical ammonium nitrate solution, said container being coated contact with it. The potential shift in the more active with a composition consisting essentially of at least 5% direction (i.e. more electronegative) is due to the elecby weight of minute carbon particles, about 5 to 20% trolytic reduction of the protective oxide film by the curby weight of an arsenic compound selected from the rent that is produced by this galvanic couple. When group consisting of arsenates and arsenites essentially in passive steel is activated there is first a steep fall of the soluble in said ammonium nitrate solution and essenpotential in the active direction; second, a less steep tially soluble in nitric acid, the balance being a binder change lasting from a fraction of a minute to several minchemically inert to said ammonium nitrate solutions. uates; and third, a steep descent to the active value (i.e. 5. The container of claim 4 wherein the solution is an complete breakdown of the passive film). The value of ammoniacal ammonium nitrate solution. the potential immediately preceding this last descent is 6. The ferrous container of claim 5 wherein the coatcalled the Flade potential. To determine the stability ing is about 0.5 to 20 mils in thickness. of the passive films produced by the coating compositions in Table I, a 2" piece of No. 12 copper wire Was immersed and physically contacted with steel coupons coated with the coating compositions in the ammoniacal solu- References Cited in the file of this patent UNITED STATES PATENTS tion of Example I inhibited with 0.15% by Weight 1,770,828 Arent July 15, 1930 NH SCN. This test procedure is far more severe than 2,366,486 Bruni Ian. 2, 1945 electrical contact and will ordinarily exceed the Flade 2,874,105 Young Feb. 17, 1959 potential and destroy passive films. Coating composi- 2,902,390 Bell Sept. 1, 1959 tion G, that is, the composition which did not contain 3,011,862 Watkins Dec. 5, 1961 

4. A FERROUS METAL CONTAINER CONTAINING AN AMMONIAAMMONIUM NITRATE SOLUTION, SAID CONTAINER BEING COATED WITH A COMPOSITION CONSISTING ESSENTIALLY OF AT LEAST 5% BY WEIGHT OF MINUTE CARBON PARTICLES, ABOUT 5 TO 20% BY WEIGHT OF AN ARSENIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF ARSENATES AND ARSENITES ESSENTIALLY INSOLUBLE IN SAID AMMONIUM NITRATE SOLUTION AND ESSENTIALLY SOLUBLE IN NITRIC ACID, THE BALANCE BEING A BINDER CHEMICALLY INERT TO SAID AMMONIUM NITRATE SOLUTIONS. 